Little white spots have just poped up under my skin on my arms and legs not itchy and don't hurt ?

Hypomelanosis: If they are very small and very white, it is probably idiopathic guttate hypomelanosis - a scary name, but it literally means "little white spots of unknown cause." the good news is that it's harmless; the bad news is that there is no reliable treatment. If you think it might be something else, let a dermatologist look at it in person.